The invention relates to a process for chromatizing zinc articles or, respectively, articles which have been electroplated with zinc in particular from acid baths, such as sulfuric acid containing plating baths.
It is known to provide zinc or articles electroplated with zinc with a passivating chromium coating. Thus it is possible to provide zinc and zinc coatings with a black chromium layer (by the so-called black chromium electroplating process) by using electrolytes which contain about 350 g./l. of chromium trioxide CrO.sub.3, 5 g./l. of barium carbonate and 5 g./l. of glacial acetic acid at high current densities of 100 to 200 amps/dm.sup.2 and low temperatures of from 12.degree.C. to 18.degree.C. The chromium coatings obtained in this manner contain 20 to 30% of chromium oxide but the process is costly and not generally applicable. Further brightening of zinc coatings may be performed by immersing the zinc plated articles in a solution of 100 g./l. of chromium acid, 10 g./l. of sulfuric acid and 5 g./l. of nitric acid. The chromium coatings obtained by this electroless chromium plating process are of a brown color. Frequently the unterlying zinc coating is attacked by this process and it is not possible to obtain chromium coatings which are resistant to wiping and scratching and which are sufficiently adhering in particular on articles being electroplated with zinc from acid baths.